Project Summary/Abstract This proposal is to request funds to validate and refine our existing CT segmentation method by (A) acquiring paired Magnetic Resonance Images (MRI) and Computerized Tomography (CT) scans from an adequately-large sample of US adults aged 40+, (B) calculating brain volumetrics and their confidence intervals from both CT and MRI scans and (C) refining the current version of our CT segmentation method so that there is a more precise correspondance between the measures derived from our CT algorithm and those derived from MRI. This supplement will allow us to exploit an exceptional and unexpected opportunity that fits within the approved scope of work. Specifically, it will allow the collection and analysis of a sample of paired CT and magnetic resonance imaging (MRI) scans from the same individuals. The opportunity to compare metrics from CT and MRI in the same set of individuals has become available at the University of Southern California (USC) after the original application was submitted, and these paired samples are ideal for advancing our project. This proposed supplement will improve our segmentations, with a target mean C_SD value 2s higher than currently available, and a target w within 5% of its true value(s) if w = 2 mm.